Pumped hydro storage plants are the most widely deployed form of large scale energy storage hydro globally. They function as a massive, grid-scale battery, using gravitational potential energy to store electricity. The basic design consists of two reservoirs at different elevations, connected by a penstock (a large pipe) and a powerhouse. When there is a surplus of electricity on the grid—typically during off-peak hours or when intermittent renewables like solar and wind are generating in excess—the plant uses this electricity to pump water from the lower reservoir to the upper one.

The real magic happens during periods of high electricity demand. When the grid needs more power, the plant simply reverses the process. The water is released from the upper reservoir, flows back down through the penstock, and turns a turbine to generate electricity. This ability to absorb and release vast amounts of power on demand makes pumped hydro storage plants indispensable for grid stability. They can provide essential services like frequency regulation and black-start capabilities, which are crucial for a reliable and resilient power system. These plants have a very long operational life, often exceeding 50 years, and a high efficiency rate of around 70-80%, making them a cornerstone of the modern energy infrastructure.